I went on the Carnival Spirit Northbound Alaska cruise last year. I had a great time. From what I hear, an aft cabin is ideal because it offers protection from the wind, a view of both sides, and the balcony is bigger. If you can get an aft cabin, you should choose a starboard balcony if your cruise is northbound and a port balcony if you're southbound. This way you'll always see land. Of course, if you're on the interior waterways, the side of the ship won't matter much. Also, If your cruise is roundtrip, it won't matter. However, no matter where you end up you'll still have a great cruise.
As for the deck, I'd say it doesn't matter too much. Just make sure your view isn't restricted by the lifeboats on deck 5. Any more questions, please ask.
Holiday Baja Mexico 8/98
Disney Wonder Bahamas 8/00
Ecstasy Baja Mexico 8/02
Carnival Spirit Alaska 7/03
Ecstasy Baja Mexico 8/03
Adventure of the Seas Southern Caribbean 12/04
Carnival Pride Mexican Riviera 8/05
Aft viewing is excellent for scenery, but poor for wildlife. BUT any wildlife viewing requires a great deal of invested time of which few passengers are willing. More important- spend time out on the front deck your glacier viewing day to see it all.
i agree with the aft of the ship. i had an inside cabin on the main deck of the carnival spirit and it was a great cabin. the motion is much less in the back of the ship and helps prevent motion sickness if you are prone to it.